Night vision systems include image intensification, thermal imaging, and fusion monoculars, binoculars, and goggles, whether hand-held, weapon mounted, or helmet mounted. Standard night vision systems are typically equipped with one or more image intensifier tubes to allow an operator to see visible wavelengths of radiation (approximately 400 nm to approximately 900 nm). They work by collecting the tiny amounts of light, including the lower portion of the infrared light spectrum, that are present but may be imperceptible to our eyes, and amplifying it to the point that an operator can easily observe the image. These devices have been used by soldier and law enforcement personnel to see in low light conditions, for example at night or in caves and darkened buildings. These devices take ambient light and magnify the light up to and in excess of 50,000 times and display the image for viewing through an eyepiece. A drawback to night vision goggles is that they cannot see through smoke and heavy sand storms and cannot see a person hidden under camouflage.
Infrared thermal sensors allow an operator to see people and objects because they emit thermal energy. These devices operate by capturing the upper portion of the infrared light spectrum, which is emitted as heat by objects instead of simply reflected as light. Hotter objects, such as warm bodies, emit more of this wavelength than cooler objects like trees or buildings. Since the primary source of infrared radiation is heat or thermal radiation, any object that has a temperature radiates in the infrared. One advantage of infrared sensors is that they are less attenuated by smoke and dust and a drawback is that they typically do not have sufficient resolution and sensitivity to provide acceptable imagery of the scene.
Fusion systems have been developed that combine image intensification with thermal sensing. The image intensification information and the infrared information are fused together to provide a fused image that provides benefits over just image intensification or just thermal sensing. Whereas typical night vision devices with image intensification can only see visible wavelengths of radiation, the fused system provides additional information by providing heat information to the operator.
FIG. 1 is a block diagram of an electronically fused night vision system 100, FIG. 2 is a block diagram of an optically fused night vision system 200, and FIG. 1A is an illustration of the fused night vision systems 100 and 200. The systems electronics and optics are housed in a housing 102, which can be mounted to a military helmet, and are powered by battery pack(s) 104. Information from an image intensification (12) channel 106 and a thermal channel 108 are fused together for viewing by an operator through one or more eyepieces 110. The eyepieces 110 have one or more ocular lenses for magnifying and/or focusing the fused image. The I2 channel 106 is configured to process information in a first range of wavelengths (the visible portion of the electromagnetic spectrum from 400 nm to 900 nm) and the thermal channel 108 is configured to process information in a second range of wavelengths (7,000 nm-14,000 nm). The I2 channel 106 has an objective focus 112 and an I2 tube 114 and the thermal channel 108 has an objective focus 116 and an infrared focal plane array 118.
As shown in FIG. 1, the I2 information is coupled to charge-coupled device (CCD) and electronics 140 and the thermal information is coupled to signal processing electronics 144. The output from the CCD and electronics 140 and the signal processing electronics 144 are inputted into mixing/display electronics 142. The analog video signal output of the mixing/display electronics 142 is coupled to displays 146 for viewing through eyepieces 110.
As shown in FIG. 2, the I2 information from the I2 channel 106 is directed to the eyepieces 110 using a beam splitter cube 230, a prism 232, and an optical relay 248. The thermal information from the focal plane array 118 is inputted into the signal processing electronics 250 and then outputted to display 246. The output of the display 246 is projected onto the beam splitters cube 230 for viewing through eyepiece 110.
The housing 102 has three knobs mechanically coupled to potentiometers 120, 122, and 124. In the electronically fused system 100, potentiometer 120 controls system on/off and display brightness, potentiometer 122 controls auto/manual gain of the thermal channel, and potentiometer 124 controls the mix of thermal and image intensification information viewable through the eyepieces 110. The on/off brightness potentiometer 120 allows the operator to turn the system on and off and control the brightness of the fused image in the displays 146, auto/manual gain potentiometer 122 allows the operator to select between manual and automatic control of the gain of the thermal channel 108, and the fusion mixing potentiometer 124 coupled to the mixing/display electronics 142 adjusts the proportional summation of the focal plane signal and the CCD signal. When the fusion mixing potentiometer 124 is rotated in one direction, the perceived percentage of I2 information viewable in the eyepieces 110 is decreased and the perceived percentage of thermal information viewable in the eyepieces 110 is increased. When the fusion mixing potentiometer 124 is turned in the opposite direction, the perceived percentage of I2 information viewable in the eyepieces 110 is increased and the perceived percentage of thermal information viewable in the eyepieces 110 is decreased. Using the on/off brightness potentiometer 120, the perceived brightness of the displays 146 can be controlled independently of the mix of I2 and thermal information in the fused image.
In the optically fused system 200, potentiometer 120 controls brightness of the thermal image, potentiometer 122 controls auto/manual gain of the thermal channel and potentiometer 124 controls the I2 channel gain. When potentiometer 120 is increased the perceived percentage of thermal information in the fused image increases and when potentiometer 124 is increased the perceived percentage of I2 information in the fused image increases. A problem with the optically fused system 200 is that two separate potentiometers must be adjusted to control the mix of I2 and thermal information in the fused image and there is no independent control of the perceived brightness of the fused image.
Fusion goggle systems have the optical axis of the thermal channel physically offset a fixed distance from the optical axis of the I2 channel. The optical axes of the thermal channel and the I2 channel are typically factory aligned such that the image from the thermal channel is fused and is aligned in the eyepiece with the image from the I2 channel when the image being viewed is at a predetermined distance, typically aligned at infinity. At distances different from the predetermined distance, parallax can cause a misalignment of the two images in the eyepiece. The parallax problem exists if the thermal channel and the I2 channels are offset in the horizontal as well as the vertical directions.
In fusion night vision systems, light entering a thermal channel is sensed by a two-dimensional array of infrared-detector elements. The detector elements create a very detailed temperature pattern, which is then translated into electric impulses that are communicated to a signal-processing unit. The signal-processing unit then translates the information into data for a display. The display may be aligned with an image combiner for viewing through an ocular lens within an eyepiece. Thermal imagers can sense temperatures ranging from −40 to +50° C. and can detect changes in temperature as small as 0.025° C. The different temperatures are typically displayed as varying shades between black and white. Depending on the location of a target and its surroundings, information from the thermal channel can obscure the information from the image intensification channel and make it more difficult to acquire and identify a target.
Night vision systems may also employ displays that may be viewed through the eyepiece. These displays, often referred to as heads-up displays, may display system information and/or scene information from an infrared sensor. Information from the display may be overlaid on the image intensification scene information and/or the infrared scene information.
Night vision systems have incorporated cameras to record battle scene information. Some night vision systems have the camera located in the optical path between the operator's eye and the vertex of the first optical element (referred to herein as the eye relief). The draw back to this approach is that the camera encroaches on the eye relief. To restore an acceptable eye relief, night vision system must be moved further from the eye of the operator. Night vision systems are intended to be portable with smaller and lighter systems being more desirable. These systems may be mounted to headgear, for example military issue AN/AVS-6 or BNVIS headgear. A heavier system that has a center of gravity far from the soldier's head results in discomfort and neck strain to the user.
Other night vision systems have attempted to digitize the image intensification scene information and combine the image intensification scene information in a display with either the system information and/or the scene information from an infrared sensor/detector. The drawback to these systems is that the resolution of the digitized imagery is reduced typically by a factor of two, which is often insufficient for most military applications.